The embodiments herein relate to methods for producing cement slurries that are resistant to fluid invasion when placed into a wellbore.
Subterranean formation operations (e.g., stimulation operations, sand control operations, completion operations, etc.) often involve placing a cement column around a casing or liner string in a wellbore. The cement column is formed by pumping a cement slurry downhole through the casing and upwards through the annular space between the outer casing wall and the formation face of the wellbore. After placement, the cement slurry develops into a gel and then cures in the annular space, thereby forming a column of hardened cement that, inter alia, supports and positions the casing in the wellbore and bonds the exterior surface of the casing to the subterranean formation. Among other things, the cement column may keep fresh water zones from becoming contaminated with produced fluids from within the wellbore. As used herein, the term “fluid” refers to liquid phase and gas phase materials. The cement column may also prevent unstable formations from caving in, thereby reducing the chance of a casing collapse or a stuck drill pipe. Finally, the cement column forms a solid barrier to prevent fluid loss to the formation, contamination of production zones, or undesirable fluid invasion into the well. Therefore, the degree of success of a subterranean formation operation depends, at least in part, upon the successful cementing of the wellbore casing.
Fluid invasion into a cement column is a known problem encountered in primary cementing operations. As used herein, the term “primary cementing” refers to the process of placing a cement column around a casing or liner string. Fluid invasion may occur before the cement slurry is cured, which may be particularly damaging, or after the cement slurry is cured. The trigger mechanism for fluid invasion may be the presence of an underbalanced pressure (e.g., the pressure of a given depth inside the cement column may be smaller than the formation pressure at that depth or nearby depths). A number of other factors may also influence fluid invasion including, but not limited to, properties of the subterranean formation and properties of the cement slurry (e.g., rheological properties). As used herein, the term “fluid invasion potential” is used to describe the tendency of fluid to invade a cement slurry or column by any mechanism. When combined with buoyancy effects, fluid invasion may result in the formation of channels within the cement column. As used herein, the term “channel” refers to a defect in the quality of cement, where the cement does not fully occupy the annulus between the casing and the formation face. The channels may result in loss of integrity of the cement column, failure of zonal isolation, and/or wellbore structural failure.
Because of the damaging effects of fluid invasion into a cement column, a number of evaluation methods have been proposed to evaluate the potential of fluid invasion, especially during the time in which the cement slurry has not yet cured. These methods, however, may be oversimplified and not properly capture multiple factors that may influence fluid invasion. As a result, the predictive capabilities of such methods may be limited.